1. Field of the Invention
The present relates to a faceplate for an electronic circuit card which reduces EMI emissions between a plurality of such circuit cards inserted in a circuit card housing. More specifically, the present invention relates to an electronic circuit card faceplate having an electrically conductive movable member which can be moved into contact with the faceplate of an adjacent circuit card inserted in the circuit card housing to reduce EMI emissions through the space between the faceplates of the adjacently inserted circuit cards.
2. Description of the Related Art
Many types of analog and digital electrical equipment produce stray electromagnetic radiation, referred to as electromagnetic interference (EMI), which is emitted into the surrounding environment. The EMI usually results from analog circuit components which oscillate at high frequencies, or from digital components which operate at high clocking or switching rates. These emissions are undesirable since they can, if sufficiently strong, interfere with the operation of radio receivers and other nearby electrical equipment. Regulations have been established for the maximum permissible EMI emissions from various types of electrical equipment, and these regulations must be taken into account when designing new equipment in which EMI might be a problem.
For some types of electronic equipment, such as telephone channel banks used to carry out analog-to-digital and digital-to-analog conversion between subscriber lines and telephone company lines, EMI reduction is difficult because of the basic design of the equipment and the practical need to allow access to the equipment by service personnel. Typically, telephone channel banks consist of rows of pull-out circuit cards, referred to as channel units, which are contained in a metal frame or housing. The housing is grounded and provides effective EMI shielding at the top, bottom, sides and back of the channel banks, but there is little shielding at the front since the frame must be left open to allow for the removal and replacement of the individual channel units. In essence, the rectangular front opening of each row or shelf in the housing serves as a slot or waveguide antenna for the electromagnetic radiation. Even though these openings are physically closed off by the channel unit faceplates when all of the channel units are fully inserted, they are electrically open since the channel unit faceplates are typically made of plastic and are essentially transparent to electromagnetic radiation.
Several approaches have been developed for reducing EMI emissions and telephone channel banks. For example, as described in U.S. Pat. Nos. 4,991,062 and 5,084,802, both to Nguyenngoc, and both incorporated by reference herein in their entirety, a resilient metal strip is fastened to the back of the channel unit (circuit card) faceplate and configured to contact the outwardly facing conductive areas of the grounded channel bank housing when the channel unit is inserted into the grounded channel bank housing. The resilient metal strip thus creates an electroconductive shunt across the shelf opening and reduces its effectiveness as a slot antenna. Other arrangements for reducing EMI emissions in telephone channel banks are described in U.S. Pat. No. 5,386,346 to Wilfred L. Gleadall, U.S. Pat. No. 5,491,613 to Eric M. Petitpierre, and U.S. Pat. No. 5,463,532 to Eric M. Petitpierre and John E. Holmes, each of which is incorporated by reference herein in its entirety.
Although these approaches are effective in significantly reducing EMI emissions in telephone channel banks, they do not provide a continuous conductive shell or Faraday shield across the shelf opening which eliminates all or essentially all EMI emissions. In particular, in all of the arrangements discussed above, an electrically open space, if not a physical space, transparent to electromagnetic radiation exists between the adjacent channel unit faceplates, through which some EMI can be emitted into the surrounding environment.
A number of approaches have been developed for reducing EMI emissions through the spaces between faceplates of circuit cards inserted into circuit card housings . For example, as described in U.S. Pat. No. 4,970,625 to Belanger et al., U.S. Pat. No. 4,903,170 to Finney et al., U.S. Pat. No. 4,872,212 to Roos et al., and U.S. Pat. No. 4,631,641 to Brombal et al., all of which are incorporated by reference herein in their entirety, as well as in published U.K. Patent Application Nos. 2263199 and 2196798, and in published Japanese Patent Application No. 4-15996, a plurality of resilient electrically conductive strips, made of a beryllium copper alloy or similar material, are connected along one of the side edges of the circuit card faceplate. Hence, when the circuit cards are inserted into a circuit card housing, the resilient electrically conductive strips on one faceplate contact the faceplate of an adjacent circuit card, thus creating an electrical interconnection between the faceplates. The electrical interconnection acts as an electroconductive shunt across the opening between the faceplates to reduce EMI emissions between the faceplates. However, several disadvantages exist with these types of electrically conductive strip configurations.
In particular, the resilient electrically conductive strips are generally sharp, and therefore can easily cut the fingers of a person installing or handling the circuit card. Also, the electrically conductive strips make installation of the circuit card more difficult, because they rub against, and can even be obstructed by, the faceplate of an adjacent circuit card during installation. An increased insertion force must therefore be applied to the circuit card to load the circuit card into the housing. This makes the resilient electrically conductive strips very susceptible to damage, especially during insertion and removal of the circuit cards. Broken metal strips can enter the interior of the circuit card housing, and create unwanted short circuits between components, which could cause damage to those or other components.
As an alternative to using resilient electrically conductive metal strips, a compressible foam coated with a fine metal mesh can be attached to one edge of each circuit card faceplates. When a circuit card is inserted into the housing, the metal mesh makes electrical contact with the faceplate of the adjacently inserted circuit card, thus creating an electroconductive shunt across the opening between the adjacent circuit card faceplates. Because the metal mesh has few, if any, sharp edges, it is much less likely to cause injury to a person handling the circuit card.
However, like the electrically conductive metal strips, the compressible foam rubs against, and can be obstructed by, the adjacent circuit card faceplate, which increases the force necessary to load the circuit card into the housing. This contact with adjacent circuit card faceplates during insertion and uninsertion also makes the compressible foam and metal mesh very susceptible to damage. Furthermore, the metal mesh typically does not conduct as well as the resilient electrically conductive strips. Therefore, the metal mesh provides a less effective EMI shield than the electrically conductive strips, while offering little advantage over the conductive strips.
Instead of attempting to provide electroconductive shunts between the individual faceplates as described above, a conductive door can be mounted to the housing. When the conductive door is in the closed position in front of the circuit card faceplates, it functions as a continuous conductive shell or Faraday shield. Although this approach would be effective in blocking essentially all EMI emissions through the spaces between the circuit card faceplates, a door of this type generally may not be desirable for cost or aesthetic reasons. Furthermore, the door in its closed position would prevent access to the fronts of the faceplates, which may be required for maintenance purposes. Due to the potential inconvenience that the door would create, it is likely that operators and technicians would fail to keep the door in a closed position to allow access to the faceplates. Also, the standard circuit card housings would require modification to accommodate this type of door. Hence, although a door of this type could be effective when used as intended, the above drawbacks make it an impractical solution.
A continuing need therefore exists for an apparatus which reduces EMI emissions through the openings between the faceplates of electronic circuit cards inserted in a circuit card housing.